The problem addressed with this invention typically arises in Synchronous Digital Hierarchy (SDH) and Synchronous Optical Network (SONET) systems in the case Of switching of parts of a network path due to a failure condition or in case of a faulty or defect part of the network path. For a deeper understanding of SDH and SONET systems, reference is made to “Understanding of SONET/SDH”, ISBN 0-9650448-2-3, Andan Publisher, New Jersey. In the before-mentioned network systems a tandem connection is intended to provide an administrative monitoring domain which is operating independent of the end to end path. Errors and faults outside the tandem connection should not be noticed in the tandem connection performance monitoring and fault management itself. However, the existing tandem connection equipment specifications in ETSI EN300 417-4-1 and ITU-T G.783 do not support this independence under all conditions especially if frame offset discontinuities occur. A frame offset discontinuity is a change of the pointer's value which defines the flexible start position of the Virtual Container within the SDH frame or an overflow or underflow of the elastic store used to adapt the ingressing data stream to the system frequency. Pointer justifications (i.e. increment and decrement operations) also change the value of the pointer but they are not frame offset discontinuities in the sense of or according to the definition as used in connection with this invention.
On every incoming frame offset discontinuity of the Virtual Container (VC) within the SDH frame, the tandem connection performance monitoring system will detect errored blocks and therefore declare one or two errored seconds (during which data are declared to be faulty or errored). Also Tandem Connection (TC) alarms may be raised.
These undesirable effects are caused by the fact that Pointer Processors (PP) are used in the network elements for the adaptation of the frequency of the incoming signal to the internal frequency of the network element and for accessing the virtual containers.
The transport of frame offset discontinuities through the network (series of pointer processors) will be fast (few bytes delay per Pointer Processor) in the virtual container signal, and much slower in the Administrative Unit (AU) or Tributary Unit (TU) (0 to 1 frame delay per pointer processor). For a certain amount of time the frame start indication, i.e. the pointer at the tandem connection sink node is not aligned with the actual phase of the virtual container. Information will be read from the wrong byte positions during this period of misalignment. This effect increases with the number of pointer processors in the chain.
The problem for tandem connection trails is introduced by the fact that there is no separate layer to transport the tandem connection information and that the path (virtual container) layer is used instead. Therefore interruptions on the path (virtual container) layer also influence the tandem connection layer.